We continued studies of cDNAs to the mammalian DNA replication proteins, Alpha-polymerase, Beta-polymerase and single-stranded nucleic acid binding protein (helix destabilizing protein). Our full-length cDNA for the binding protein was subcloned in an expression vector and the protein was over produced in E. coli and isolated in mg quantities. The amino acid sequence of this protein was identical with that of the protein deduced from the cDNA and with the sequence of the A1 protein purified from mammalian hnRNP particles. Protein-nucleic acid binding studies revealed that the C-terminal domain of the protein is critical for tight binding. The sequence of our cDNA to rat Beta-polymerase was determined. Properties of the sequence 5' of the first ATG in the cDNA suggested that this ATG is the initiation codon for synthesis of the 318 amino acid protein encoded by the single long open reading frame of the mRNA. This cDNA was used to probe a cDNA library of poly A+ RNA from a human cell line and an analogous human cDNA was isolated and sequenced. The human and rat cDNAs were 90% homologous and the proteins deduced from the two open reading frames were similar (95% matched residues). Our rat cDNA to Alpha-polymerase (1.2 kb) was sequenced and found to contain only a short open reading frame, presumably corresponding to the C-terminal end of the Alpha-polymerase catalytic polypeptide. Antibody against a synthetic oligopeptide with this sequence reacted with purified Alpha-polymerase and immunoprecipitated Alpha-polymerase from solution. A panel of longer cDNAs was obtained from a library to human poly A+ RNA using our antibody to the Hela Alpha-polymerase, catalytic polypeptide. These various human and rat cDNAs are now being used as probes to study expression of mRNAs, to identify polymorphisms in genomic DNA, and to conduct chromosomal localizations.